The invention concerns tire covers with carcass reinforcement overlaid radially by a crown reinforcement consisting of at least two plies of reinforcement elements parallel to one another within each ply and crossed from one ply to the next, which for an acute angle with the circumferential direction.
Patent application EP-A-672 547 describes a tire cover having no bead wire, but which comprises an annular element whose tensile strength is substantially less than would be necessary for a bead wire in a known tire of the same size, and at least one bead reinforcement armature consisting of two reinforcing layers in contact with or close to the annular element, the combination of these layers having a tensile rupture strength at least equal to that necessary for a bead wire in a known tire of the same size. The mechanical strength of the bead reinforcement armature is thus the essential contribution to the mechanical strength of the bead reinforcement combination consisting of the annular element and the said armature, so that the combination can replace the bead wire of a classical tire.
The example described in the European application shows a bead reinforcement armature consisting on the one hand of two adjacent reinforcement plies with their respective radially lower ends in contact with the annular element and extending radially above the said element.
An additional reinforcement ply surrounds the annular element and the first two plies, so forming two elementary layers applied on either side against the first layers which are accordingly sandwiched between the elementary layers, such that the four layers in contact are practically parallel to one another.
The bead reinforcement armature can also comprise only two reinforcement layers, for example such as the fist plies described above with the ply turned up over itself to form two layers. The first two layers can be obtained by folding a single ply, and this gives a bead reinforcement armature with four layers, the said layers formed by turning up over themselves two respective plies.
Cases have also been envisaged in which the carcass ply is positioned between the reinforcement layers with the carcass ply wrapped directly around the annular element and the two reinforcement plies wrapped around the carcass ply and the annular element.
The reinforcement elements of one reinforcement layer are crossed with those of the adjacent or axially closest reinforcement layer(s), in other words the respective elements of two layers adjacent or axially closest to one another are not parallel to one another.
The best solutions from the endurance standpoint seem to be those in which the reinforcement layers of cords or cables crossed from one layer to the next are obtained by turning up a single ply over itself. The folding or turning up of one or two superimposed plies that make angles between 0xc2x0 and 10xc2x0 with the circumferential direction is a delicate operation. The production of a tire comprising beads reinforced in the manner described above poses additional manufacturing problems too, because obtaining a precise position of the various bead elements is unsatisfactory since the said elements are too disarranged during the stages of building up, shaping and curing; this results in irregular quality and properties in the series of tires produced.
The purpose of the invention is to propose a tire cover of the type described above which can be fabricated more easily and with more reliable regularity of the tire properties.
The tire according to the invention comprises a carcass reinforcement overlaid by a crown reinforcement, itself covered radially by a tire tread connected to two beads by two side walls, each bead having no anchoring bead wire and the said carcass reinforcement extending from one bead to the other, and is characterized in that each bead is reinforced by an armature comprising at least two plies turned up over themselves to form four reinforcement layers axially close to one another, each of the said layers comprising reinforcement elements parallel to one another within each layer and making with the circumferential direction an angle xcex1such that 0xc2x0 less than xcex1,xe2x89xa610xc2x0; the two axially innermost layers being composed of elements parallel to one another from one layer to the next and the two layers axially at the center of the armature composed of elements crossed from one layer to the next, such that the combination of reinforcement layers has a tensile rupture strength measured in the circumferential direction at least equal to the strength required for a bead wire in a known tire of the same size, and the reinforcement layers have their radially outside ends arranged in the bead at different heights, such that any annular strip of the bead reinforcement armature comprising at least two layers of reinforcement elements and located between two parallels of the tire is formed of at least two layers of reinforcement elements crossed from one layer to the next.
The above structure makes it possible when fabricating the tire to omit an annular element whose longitudinal tensile rupture strength is substantially less than would be necessary for a bead wire in a known tire of the same size, the axis of the said annular element being the rotation axis of the tire. On the other hand, the presence of the said element advantageously fulfils another function: that of filling the radially lower part of the bead so as to confer upon that part an axial width compatible with the width of the rim seat on which the tire will be fitted, and compatible with the rim tightness required to prevent any rotation on the rim.
Although the turn-up of the carcass reinforcement and the main part of the carcass reinforcement corresponding to it may be located axially between the two axially innermost reinforcement armature layers and the two axially outermost layers of the said reinforcement, it is advantageous for the carcass reinforcement to be wrapped around the radially lower part of the bead reinforcement armature, the part formed by turning up the two reinforcement plies. The turn-up of the carcass reinforcement can also be inserted between the two axially outermost layers, while the main portion of the carcass reinforcement corresponding thereto is inserted between the two axially outermost layers.
It is also advantageous for the section which covers the bead on the outside at least axially, the part of the said protective section or layer intended to come into contact with the rim at least in the area of the rim flange when the tire is fitted, to be made from a rubber mix which, in the vulcanized state, has a dynamic elastic loss modulus Gxe2x80x3 of less than 1, Gxe2x80x3 being expressed in MPa (megapascals), the said modulus being measured at 10% shear, at a temperature of 50xc2x0 C. and with a frequency of 10 Hz, and the said section being at most 2 mm thick.
The term xe2x80x98reinforcement elements of the turned-up plies of the bead reinforcement armaturexe2x80x99 should be understood to mean cords as well as cables. A cord may consist of a single monofilamentary or multifilamentary strand, whereas a cable may or may not have a central core.
The material constituting the reinforcement elements may vary, being for example metallic and especially steel, but is preferably a natural or synthetic textile such as a cellulose, polyester, or aliphatic and preferably aromatic polyamide material, in particular aramide cords; it may also be mineral, for example glass or carbon, and each cable can comprise several of the materials mentioned (these being known as mixed cables).